Lens forming methods are described in application Ser. No. 07/779,317, filed Oct. 18, 1991, which was a continuation in part of application Ser. No. 07/740,946, filed Aug. 6, 1991, which was a continuation in part of application Ser. No. 07/594,136, filed Oct. 10, 1990, which was a continuation in part of application Ser. No. 07/446,151, filed Dec. 5, 1989, which was a continuation in part of application Ser. No. 07/422,399, filed Oct. 12, 1989, application Ser. No. 07/339,217, filed Apr. 17, 1989, and application Ser. No. 07/190,856 filed May 6, 1988, and of application Ser. No. 07/114,962, filed Oct. 30, 1987, now U.S. Pat. No. 4,873,029, all of which are hereby incorporated by reference.
An excellent summary of conventional compositions, methods, and apparatus for making plastic lenses is provided in U.S. Pat. No. 4,879,318, to Lipscomb et al. (All patents and other documents mentioned herein are incorporated by reference as if reproduced in full below.) In particular, Lipscomb et al. teach that conventional lens forming compositions include diethylene glycol bis(allyl)-carbonate (DEG-BAC), and one or more monofunctional or multifunctional acrylate monomers that can be thermally or radiation cured to produce hard, tough, clear, and strain-free optics. Photoinitiators may be selected from the group consisting of 2-hydroxy-2-methyl-1-phenylpropan-1-one and 1-hydroxycyclohexylphenyl ketone. The monofunctional or multifunctional acrylate monomers may be selected from tetrahydrofurfuryl acrylate (TFFA), trimethylol propane triacrylate (TMPTA) and tetrahydrofurfuryl methacrylate (TFFMA).
Lipscomb et al. found that, by mixing DEG-BAC with additives or comonomers, its cracking tendency could be decreased. The rate of polymerization was increased by combining an acrylate containing group with DEG-BAC polymer; suitable acrylate groups include tetraethylene glycol diacrylate (TTEGDA), tripropylene glycol diacrylate (TRPGDA), trimethylol propane triacrylate (TMPTA), tetrahydrofurfuryl methacrylate (TFFMA), and tetrahydrofurfuryl acrylate (TFFA).
TTEGDA tends to increase the overall rate of polymerization, and tends to reduce the amount of yellowing in a finished lens, but TTEGDA also tends to increase lens cracking. TRPGDA also increases the rate of polymerization, while TMPTA and TTFMA tend to prevent the development of patterns and fringes in finished lens. TFFA tends to reduce cracking and the development of patterns in the finished lens, and also tends to reduce the degree to which a lens sticks to a mold; more than 25% by weight of TFFA should not be included in a DEG-BAC lens, since a proportion greater than 25% tends to decrease the hardness of the finished lens.
Convective striations or optical inhomogeneity, referred to as "patterns" or "wavy patterns," are defects which usually occur during early stages of polymerization, during which the resin composition is transformed from a liquid to a gel state; once patterns form, they are almost impossible to eliminate. Further, when gelation occurs, there is a rapid temperature rise due to the exothermic polymerization reaction, and, in positive lenses, the temperature can quickly reach 85.degree. C., and lead to lens fracture. The exothermic reaction also causes an increase in the rate of polymerization, which in turn causes a further increase in temperature. Therefore, heat exchange with the surroundings must be efficient enough to avoid uncontrolled rapid polymerization, which can lead to the appearance of thermally caused striations and/or breakage. Thus, the gelation point is a critical phase of the polymerization reaction, since the rate of polymerization increases rapidly at the gelation point.
Lipscomb et al. teach that the best quality lenses result from a smooth reaction process which is not too fast or slow. The heat generated by the exothermic polymerization reaction must not be generated faster than it can be exchanged with the surroundings. The resin of Lipscomb et al. is cured with ultra-violet light having the intensity adjusted to control the reaction rate. Blockage of shorter wavelengths below 320 nm was found to be absolutely essential by Lipscombet al., since the full intensity of ultra-violet light striking the glass mold caused breakage of the mold.
An alternate embodiment of Lipscomb et al. replaces DEG-BAC with the monomer 4,4'-isopropylidene diphenol diallyl carbonate (bisphenol A diallyl carbonate) in admixture with faster reacting monomers, such as trimethylol propane triacrylate (TMPTA), hexanediol dimethacrylate (HDDMA), tetraethylene glycol diacrylate (TTEGDA), tripropylene glycol diacrylate (TRPGDA), and styrene (generally, compounds containing acrylate groups polymerize much faster than those containing allyl groups).
Since Lipscomb et al. is directed to forming whole lenses in a mold cavity surrounded by a gasket, it is necessary to carefully control the cure temperature to avoid cracking and to obtain uniform curing throughout the thickness of the lens. Lipscomb et al. rigorously control cure temperature by actively cooling the mold assembly during the cure process in order to dissipate the heat of polymerization, and thus avoid mechanical deformation or cracking. However, curing at a low temperature results in a lower glass transition temperature and a lower degree of cross-linking, which leads to lenses having a lower scratch resistance, lower durability, and a lower Barcol hardness. Further, Lipscomb et al. avoid the formation of a gradient of cross-link density through the lens, which can occur by having an accelerated rate of cure at the surface; formation of such a hard surface is actually beneficial to a final optic, but Lipscombet al. are forced to forego this benefit because of the necessity for curing resin layers having a thickness of 5 mm or greater. The resins of Lipscomb et al. are formed of low viscosity monomers, and, when cured, form optics having relatively low glass transition temperatures, even though they may reach relatively high levels of cross-link density.
Various other methods for casting monofocal, bifocal, progressive, and astigmatic correction lenses are known. Generally, all of these methods involve curing of a suitable resin in a mold. In all cases, casting of finished ophthalmic lenses (in contrast to a semi-finished blanks) over a wide range of prescriptions, requires the use of some type of restrictive apparatus, which holds the mold form(s) in fixed spatial relationship with respect to one another (e.g., gasket or edge fixture). In some cases, a semifinished or finished preformed optic (also known as a "blank," "preformed optic," "preformed blank," "single vision optic", etc . . . ) forms part of the mold assembly, and becomes part of the finished optic. The various prior art mold apparatus either adjust for the shrinkage of the resin through the use of a gasket or edge fixture, or restrict the range of optical prescriptions which can be achieved by forming a lens by molding.
For example, U.S. Pat. No. 4,190,621, to Greshes, discloses the formation of a bifocal lens on a previously formed blank lens, by casting the optic between a mold and a preform. The preform is placed in fixed vertically spaced relationship to a lower mold in a retaining support apparatus. The support apparatus holds an upper mold or preformed blank a few thousandths of an inch (greater than or equal to 0.025 mm) above the lower mold, and thus maintains a separation between the mold and the upper mold or preformed blank; an optical resin material is placed in the lower mold, such that the upper mold or preformed blank causes displacement of the resin material, so that the resin extends between the overlapping surfaces of the molds to form a desired lens configuration (prescription).
In the absence of the support apparatus, the upper mold or preformed blank of Greshes would sink down into the resin, so there would not be a sufficiently thick resin layer between the preformed blank (or upper mold) and the lower mold to form an acceptable optic. Thus, in order to form lenses of differing prescriptions, numerous different supports are required, which provide for differing separations between the lower mold and the upper mold or preformed blank. The apparatus and method of Greshes are incapable of forming plus or minus power progressive lenses which necessitate curing a thick (greater than 1.0 mm) resin layer at or about the progressive addition area, while maintaining an acceptable edge thickness.
There are numerous other ophthalmic lens forming methods disclosed in the prior art. For example, U.S. Pat. No. 3,946,982, to Calkins, utilizes two mold portions held together by a gasket; one of the mold portions includes a bifocal segment. A liquid resin is injected into the cavity formed between the two molds and cured. U.S. Pat. No. 4,873,029, to Blum, discloses a thermal casting method, which involves the use of either two molds and an intervening resin layer sealed into a mold assembly by a gasket, or a mold and a single vision wafer (preform) sealed into a mold assembly by a gasket. It is undesirable to use gaskets, since gaskets impose critical limitations on "point of use" or "in-office" optics casting processes, such as (1) a very large number of gaskets have to be stocked in order to cast on demand a practically useful range of prescriptions, (2) it is more difficult to perform the assembly and the filling process of a molding apparatus using gaskets, (3) gaskets impose a limitation on the relative size of a preformed optic or blank and a mold (e.g., a smaller preform can not be used with a larger mold), and (4) gaskets cause shadowing at the lens edges or leave the edges uncured in a photocuring process.
U.S. Pat. No. 4,623,496, to Verehoven, discloses a method of casting aspheric lenses from a spherical substrate in which the substrate matrix can be positioned without a guiding mechanism, and uses a weight to press a substrate into resin placed in a mold to form a very thin resin layer. Verehoven achieves this by severely restricting the range of prescription (i.e., thickness of the cast polymer layer), and by limiting the range of radii of curvature, and hence the spherical power of the substrate. In order to achieve the desired separation between the substrate and the matrix, a series of inflection points is situated in a circle at the point where the substrate and matrix are closest to each other. Verehoven accepts limitations on the range of radii of curvature and power of the resulting lenses in order to minimize problems due to shrinkage of the resin during the cure process. Further, due to the thinness of the cast layers of Verehoven around the infection points, the resulting lenses will not evenly tint.
U.S. Pat. No. 2,339,433, to Staehle, discloses the casting of monofocal or multifocal lenses by adding a correction to a molded plastic lens; a thin, relatively uniform layer of plastic is cast onto a single vision optic in order to form a monofocal optic, and the resin layer is then thermally cured. The resin must have the same refractive index as the preformed plastic lens. It is not possible to produce bifocal or multifocal lenses having a wide range of prescriptions with Staehle's method, and the method is only useful for adding a thin, relatively uniform layer of plastic to a single vision optic in order to form a monofocal optic.
U.S. Pat. No. 3,248,460, to Naujokas, discloses a method for casting multifocal lenses, in which a base blank is used, having a curvature only about half of the curvature of the predetermined power of the final optic. The limitation in the curvature of the preformed lens blank is necessitated by the need to control variation in the thickness of the liquid resin layer to be cured in order to form the final optic over the entire optic surface.
U.S. Pat. No. 4,166,088, to Neefe, discloses formation of plastic lenses by curing a mixture of a liquid monomer and a photosensitive initiator in a mold cavity formed between a pair of spaced apart molds, and uses ultra-violet light to cure the resin. U.S. Pat. No. 4,298,005, to Mutzhas, discloses a suitable ultra-violet source for curing of optical resins. U.S. Pat. No. 3,038,210, to Hungerford et al., and U.S. Pat. No. 3,222,432, to Grandperret, disclose methods and apparatus for thermally curing lens forming materials in mold cavities. Likewise, U.S. Pat. No. 4,758,448, to Sandvig et al. discloses a method for forming thin (0.5 to 50 microns), optically clear abrasion resistant coatings on optical surfaces.
Australian Patent Document No. 80556/87, to Squires, discloses a method of casting a layer of polymeric material onto the surface of a single-vision molded ophthalmic lens, which includes roughening the front surface of the single vision lens to enhance subsequent adhesion of the cast-on polymeric material, and removal of unwanted cylinder by placing a weight onto the periphery of a single-vision lens, resting in a mold of correct curvature. A small quantity of catalyzed monomer is placed between the single-vision lens and the mold, which are held in spaced relationship to one another by a jig, and the monomer is cured by either heat or UV radiation. Unwanted astigmatism is removed by subjecting the preform to a one-half kilogram load during curing, and, following curing, by separating the multifocal lens from the mold at about 60.degree. C.
Prior art optical lens casting methods either require the use of a gasket surrounding a mold to retain the lens forming resin, or utilize a method and resin which is incapable of forming the full range of prescriptions required by opticians; in particular, the prior art does not disclose a resin or method of using same, which is suitable for forming high quality, aspheric or spherical single vision, multifocal, or progressive optics over a wide range of prescriptions and diameters in a fast, simple manner from a semi-finished or finished preformed optic.
Thus, there is a need for new resin formulations for use in casting of plastic optics and methods of forming plastic optics therefrom. In particular, there is a need for a fast and simple method for forming plastic optics which have a refractive index greater than or equal to 1.48 and abrasion or scratch resistance higher than 1.0 (as determined by a Bayer abrader and a hazemeter). There is also a need for an optical resin and method of using same for fast and easy formation of aspheric or spherical single vision, aspheric, multifocal, or progressive optics over a wide range of prescriptions from a finished or semi-finished blank or preform. Further, there is a need for a process which can utilize optical preforms of widely ranging size and/or shape which will be compatible with standard optic molds, and which will quickly form plastic lenses which will meet or exceed ANSI standards for ophthalmic lenses and/or which are optically functional and cosmetically acceptable.